On Tuesday, a Ukrainian Sukhoi Su-27 fighter jet crashed during a joint NATO-Ukrainian military exercise, claiming the lives of both pilots. Although their identities have not yet been released, earlier reports suggest that one of the killed pilots was an American Air National Guard officer.

While information is still limited, the accident presumably took place during flight operations in support of NATO’s Clear Sky 2018 exercises. The crash was reported first on social media by Ukrainian military officials before being picked up by local news outlets.

“Today, around 17.00 local time [14:00 GMT], a Su-27 plane crashed near the village of Ulaniv [between the towns of Berdychiv and Khmil’nyk] during a combat training flight. Search and rescue activities are being carried out,” the Ukrainian press service said on Facebook.

“We regret to inform that, according to the search and rescue team, the bodies of two pilots have been found: one is a soldier of the Air Force of the Armed Forces of Ukraine, while another one is a soldier of the Air Forces of the U.S. National Guard.”

These statements present some interesting questions about the circumstances of the crash: The Su-27 is a single-seat fighter. The Su-30, on the other hand, is the two-seater variant. This, combined with the notion of an American pilot behind the stick of Ukrainian fighter, has led some to wonder if a second aircraft may have been involved in the crash—such as an American F-15, which are also reportedly operating in the area.

Nearly a thousand participants from nine different nations are joining in Clear Sky 2018. Although Ukraine is not yet a member of the NATO alliance, these exercises are aimed at cultivating a higher level of interoperability between the air forces of NATO nations and Ukraine, ostensibly to serve as a deterrence for the continued Russian expansion into the region.

In 2014, Russia annexed Crimea after civil unrest erupted in Ukraine following a political crisis. Since then, Moscow has also supported separatist forces along Ukraine’s Eastern border.

No details have been released regarding what may have caused the accident or why neither pilot ejected from the aircraft. The Su-27, which carries the NATO designation of Flanker, is widely considered to be among the most capable fourth-generation fighters in service. Developed by the Soviet Union in the 1980s, the Su-27, along with its carrier variant the Su-33, continue to serve in many countries, including Russia.